In the past, the strongest commercially produced permanent magnets were made from sintered powders of SmCo.sub.5. Recently, even stronger magnets have been made from alloys of the light rare earth elements, preferably neodymium and praseodymium, iron and boron. These alloys and methods of processing them to make magnets are described in U.S. Ser. Nos. 414,936 (filed 9/3/82), 508,266 (filed 6/24/83) and 544,728 (filed 10/26/83) to Croat; 520,170 (filed 8/4/83) to Lee; and 492,629 (filed 5/9/83) to Croat and Lee, all assigned to General Motors Corporation.
Sources of the rare earth (RE) elements, atomic nos. 57 to 71 of the Periodic Chart as well as yttrium, atomic no. 39, are bastnaesite and monazite ores. Mixtures of the rare earths can be extracted from the ores by several well known beneficiating techniques. The rare earths can then be separated from one another by such conventional processes as elution and liquid-liquid extraction.
Once the rare earth metals are separated from one another, they must be reduced from the oxides to the respective metals in relatively pure form (95 atomic percent or purer depending on the contaminants) to be useful for permanent magnets. In the past, this final reduction was both complicated and expensive, adding substantially to the cost of rare earth metals.
Both electrolytic and metallothermic (non-electrolytic) processes have been used to reduce rare earths. The electrolytic processes include (1) decomposition of anhydrous rare earth chlorides dissolved in molten alkali or alkaline earth salts, and (2) decomposition of rare earth oxides dissolved in molten rare earth fluoride salts.
Disadvantages of both electrolytic processes include the use of expensive electrodes which are eventually consumed, the use of anhydrous chloride or fluoride salts to prevent the formation of undesirable RE-oxy salts (NdOCl, e.g.), high temperature cell operation (generally greater than 1000.degree. C.), low current efficiences resulting in high power costs, low yield of metal from the salt (40% or less of the metal in the salt can be recovered). The RE-chloride reduction process releases corrosive chlorine gas while the fluoride process requires careful control of a temperature gradient in the electrolytic salt cell to cause solidification of rare earth metal nodules. An advantage of electrolytic processes is that they can be made to run continuously if provision is made to tap the reduced metal and to refortify the salt bath.
The metallothermic (non-electrolytic) processes include (1) reduction of RE-fluorides with calcium metal (the calciothermic process), and (2) reduction-diffusion of RE-oxide with calcium hydride or calcium metal. Disadvantages are that both are batch processes, they must be conducted in a non-oxidizing atmosphere, and they are energy intensive. In the case of reduction-diffusion, the product is a powder which must be hydrated to purify it before use. Both processes involve many steps. One advantage of metallothermic reduction is that the yield of metal from the oxide or fluoride is generally better than ninety percent.
Processes involving RE fluoride or chloride require pretreatment of the RE-oxide to create the halide. This additional step adds to the end cost of rare earth metals.
With the invention of light rare earth-iron permanent magnets, the demand for low cost, relatively pure, rare earth metals rose substantially. However, none of the existing methods of reducing rare earth compounds showed much promise for reducing the cost or increasing the availability of magnet-grade metals. Accordingly, it is an object of this invention to provide a new, efficient and less costly method of producing rare earth metals.